The present invention has reference to bleaching, especially with chlorine, of cellulose pulp obtained by delignification of vegetable fiber containing raw material such as wood, bamboo, bagasse, straw, and reeds.
Conventionally, such chlorine bleaching is performed at a pulp concentration of about 3-4 percent fibers or solids, i.e. at relatively low concentration of fibers in a watery suspension. This concentration has by experience been found most suitable, since chlorine has a very rapid initial reaction with pulp, whereby the main part of the chlorine reacts directly after the introduction. During such bleaching there is a risk that certain parts of the pulp are overchlorinated and other parts underchlorinated, but due to a relatively large water quantity an effective "mixing-in" and a relatively even bleaching result are possible. Chlorine can be added as gas or as gas dissolved in liquid, so called chlorine water.
As a process it is also known to bleach pulp with chlorine at higher concentrations, e.g. 10 percent, which generally speaking is common in treatment stages in modern bleach plants. If chlorine gas is to be mixed into pulp of 10 percent concentration, there are problems associated with proper mixing-in of the gas. If, on the other hand, the chlorine is to be added as chlorine water, the liquid quantity will be relatively large with the normal solubility of chlorine in water, and the pulp will be diluted to a concentration far below the desired 10 percent concentration.
According to the present invention it is possible by using chlorine water to bleach pulp at about 10 percent solids concentration. It has been found that the liquid quantity which is necessary in order to dilute pulp of about 30-40 percent solids concentration to about 10 percent solids concentration corresponds to the amount of liquid which contains a normal chlorine charge therein for bleaching unbleached sulphate pulp or the like. According to the present invention, pulp is dewatered by conventional dewatering apparatus, such as certain types of presses, to obtain a solids concentration of 30-40 percent. The concentrated pulp is then treated in a vessel with a first treatment liquid having between 3 and 10 grams of chlorine gas dissolved therein, corresponding to a charge of 20 to 80 kilograms of chlorine per ton of pulp, a normal chlorine charge treatment. The solids concentration of the pulp after treatment is then 6-15 percent, preferably about 10 percent.
According to another aspect of the present invention the pulp is first treated with a certain quantity of C10.sub.2 dissolved in liquid, preferably with a quantity corresponding to 2-8 kgs per ton pulp calculated as active chlorine, preferably in the same treatment vessel. The treated pulp may be transported to a retention vessel and retained therein for a predetermined period of time in order to reduce the amount of residual chlorine in the pulp for further stages of treatment thereof.